Field of Invention
The present invention relates to an image forming apparatus capable of correcting image formation positions.
Description of Related Art
Conventionally, image forming apparatuses have been proposed which can adjust the image formation positions on the front and back sides of a sheet in order that images printed on the both sides are aligned with each other. In a type of such image forming apparatuses, the positional displacement between the front and back sides is obtained, for example, by printing position adjustment marks on the front and back sides of a sheet, reading the marks of the sheet with a scanner device to obtain the positional and orientation displacement of the sheet as reading errors, performing image process in order that images on the front and back sides are aligned based on the errors, and obtaining the positional displacement between the front and back sides with reference to the processed image data (for example, refer to Japanese Patent Published Application No. 2005-91823).
Another type has been proposed which is provided with a size detecting unit for detecting a paper size by detecting the edges of a sheet, and capable of detecting the paper sizes before and after expansion of the sheet due to thermal fixing, and determining the reduce/enlarge ratio of printing in accordance with the difference between the paper sizes (for example, refer to Japanese Patent Published Application No. 2009-42461).
However, in the case of the above conventional image forming apparatuses, the accuracy of positional adjustment may be deteriorated by occurrence of magnification error (incorrect recognition of slight variation in paper size between when reading the front side of a recording sheet and when reading the back side of the recording sheet) due to the differential conveying speed of the recording sheet or the like while the scanner device is reading the position adjustment marks which are printed.
More specifically explaining, for example, there is a one-path scanning system provided with scanner devices on the front side and the back side of a recording sheet respectively on a transfer route through which the recording sheet is conveyed. In this system, the position adjustment marks are read by the two scanner devices while the recording sheet is passed through the transfer route. However, when there is a difference in the speed of the recording sheet between when the front side of the recording sheet is passed through the scanner device and when the back side of the recording sheet is passed through the scanner device, magnification error occurs as described above. Also, even when there is no difference in the speed of the recording sheet, slight magnification error may occur because different scanner devices have different characteristics and so forth.
Furthermore, for example, there is a flat head scanner capable of reading information of a recording sheet on a document reading glass surface by moving a scanner device in relation to the document reading glass surface. This flat head scanner first is moved to read the image information of one side of a recording sheet placed on the glass surface, and then moved again to read the image information of the other side of the recording sheet after reversing the recording sheet. Even in the case of this flat head scanner, magnification error may occur due to different moving speeds between when the scanner device moves to read one side and when the scanner device moves to read the other side.
As discussed above, in the conventional image forming apparatus, the accuracy of adjusting the position of images on the front and back sides is deteriorated under the influence of magnification error.